1. Field of the Invention:
This invention relates to new varnish polyisocyanates having an isocyanurate or urethane structure blocked with blocking agents from a mixture of 2-methyl-1,5-diisocyanatopentane or 2-ethyl-1,4-diisocyanatobutane or 3-isocyanatomethyl-3,5,5-trimethylcyclohexylisocyanate or a mixture thereof, and a process for the production of these new varnish polyisocyanates as well as a process for the use thereof as an isocyanate component for the production of thermosetting polyurethane powder varnishes.
2. Description of the Prior Art:
Polyurethane powder varnishes on the basis of partially or completely blocked polyisocyanates or polymers containing hydroxyl groups whose softening point is above 40.degree. are state-of-the-art and are frequently described in the literature, e.g., in the DE-OSS 21 05 777, 25 42 191, 27 35 497, 28 42 641, 30 04 876, 30 39 824, or 31 28 743.
Polyurethane powder varnishes essentially consist of a component containing hydroxyl groups and a polyisocyanate whose NCO groups are partially or completely blocked with a blocking agent, so that the OH:NCO polyaddition reaction cannot take place at temperatures below 140.degree. C. Only after heating to temperatures .gtoreq.150.degree. C. is it possible to cross-link such powder varnishes to a coating film within a relevant time while setting free the blocking agent and reacting the OH groups with the NCO groups.
Not all polyisocyanates are suitable for such purposes and in view of the application as powder materials, there are quite a number of restrictions, especially if the polyisocyanates, partially or completely blocked with the usual blocking agents, themselves have characteristics that make grindability more difficult, so that their use in powder varnishes is greatly limited if not impossible, even though their varnish technical as well as chemical and physical characteristics may be very good.
Thus, normally the isocyanato-isocyanurates, such as for example hexamethylenediisocyanate (short: HDI), are liquid as are their reaction products with blocking agents, as described in EP 0 047 452, corresponding to DE-OS 30 33 860, from page 2, line 29 to page 3, line 6, and cannot be used in powder varnishes. An exception is the isocyanato-isocyanurate of 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate, also called isophoronediisocyanate, short IPDI. It is solid and has a melting range of 102.degree.-104.degree. C. As is further shown in EP 0 047 452, products can be produced by mixed trimerization of these two polyisocyanates (HDI/IPDI), which in their blocked form, cf. page 8, lines 16-21, are suitable for the production of polyurethane powder varnishes even though this has not been proved by experiments. It is therefore possible to vary the melting points by means of the mole ratio of the diisocyanates. Moreover, among other things the improved solvent compatibility and flexibility at low temperatures are pointed out, cf. page 3, lines 19-21.
Furthermore, in practice the thermal and oxidative stability are of decisive importance, so that thermosetting polyurethane baking enamels do not show any yellowing phenomena at temperatures below those required for hardening the coating films, since otherwise, in case of technical trouble (e.g., assembly-line stoppage), during the thermosetting process more or less large quality reductions (discolorations) must be expected. In the past, this has not always been guaranteed.
Therefore, a need clearly continues to exist for polyisocyanates which, although partially or totally blocked, are suitable for use as cross-linking agents in polyurethane powder varnishes and which are susceptible to grinding. It is further required that these polyisocyanates be thermally and oxidatively stable and not contribute to yellowing of the thermosetting polyurethane baking enamels.